The changes, some of which take effect July 2, come amid an expansion of National Public Radio programming on WGBH, including additional broadcasts of “Marketplace” and extending “Morning Edition” to four hours per weekday.

But what the station is calling “a new focus on jazz” amounts to a serious downscaling of jazz programming on Boston radio, where Jackson and Schwartz have been mainstays for three decades, exposing listeners to artists old and new while promoting concerts and other events vital to the local jazz scene.

“Jazz on WGBH With Eric Jackson” will no longer run from 8 p.m. to midnight Monday through Thursday, airing instead from 9 p.m. to midnight Friday through Sunday. Schwartz’s Friday evening jazz show is disappearing altogether, and he will no longer produce live performances for Jackson’s show.

The date for these changes hasn’t been set, according to station managing director Phil Redo, who broke the news to Jackson and Schwartz Tuesday. By Wednesday it was buzzing around social media websites and jazz circles.

“That’s tragic,” said pianist Danilo Perez, who is on the faculty at Berklee College of Music, reached by phone in Colorado. “In a culture where we are so much in need of hope and optimism, that’s what jazz is all about. As long as people listen to radio, it’s crucial to have jazz [featured] there.”

Jackson, who celebrated 30 years on air last spring, said the station “has been moving in that direction for a couple of years.” A month ago he and Schwartz heard their shows would be cut one hour apiece, he added, but moving his show to weekend nights only was “a total surprise.”

To the local jazz community, “this is major,” Jackson said. “The music has always been there in the evening. To put it on the weekends at 9 p.m., when families won’t necessarily be listening together, is not the same thing.”

Live interviews and shows featuring a single artist may no longer fit the format, he said. But he added: “I still love doing radio, and Boston still needs jazz radio, because jazz is a major part of American culture.”

Schwartz, who’s been on the local airwaves for nearly 27 years, said he felt change was imminent a couple of years ago, when WGBH shifted its classical programming to WCRB-FM (99.5).

“It wasn’t a total surprise, but it is a loss,” he said. “The station is losing a consistent format spread across the week. And the Boston jazz community is losing an important venue for musicians to promote their events.”

The moves could have a negative impact on WGBH membership, Schwartz said, because WGBH Jazz Club members have access to studio concerts, which may no longer be produced.

Redo acknowledged Wednesday that by shrinking Jackson’s show, WGBH is inviting blowback. “You can’t move from 20 hours a week to nine and call it enhancement,” he said. “But as we expand [programming], we run into real estate issues. Clearly, ‘GBH has jazz in its DNA. It was never going away.”

The moves come as WGBH fights WBUR for public radio supremacy in Greater Boston, a battle that impels his station to program more public affairs shows.

Recommitting to Jackson’s show, and promoting it more effectively, should help preserve its place in that larger effort, Redo said.

In the most recent ratings period, Jackson’s weeknight show drew an average of 14,600 listeners in Greater Boston, ranking 20th among 30 shows in its time slot.

As news spread Wednesday, many in the local jazz community reacted with shock and dismay. On Facebook, a “Save Eric in the Evening” page — a reference to the show’s former title — sprang up, elicited responses ranging from sadness to outrage.

Saxophonist Ken Field said Wednesday that Jackson’s renown among artists from New England and around the world make him more than just another DJ. “Reducing his airtime is a step in the wrong direction, for people in Boston and people outside Boston,” said Field, who lives in Cambridge.

Field said he questions whether people will continue to support WGBH if it drops shows like these for more talk and news.

“I don’t know how programming is determined there, but this is very sad for listeners and the jazz community in general,” said Newport Jazz Festival producer Danny Melnick. “All of us work hard together — musicians, agents, club owners, media, presenters. We’ve relied on Steve and Eric as active parts of that community.”

Ted Kurland, who manages several local jazz artists, said he, too, was saddened by the news, but not entirely surprised. “We’ve seen a huge reshuffling of music on terrestrial radio,” he said. “Entire jazz stations are calling it quits.”

Through websites such as Pandora, jazz fans can effectively become their own broadcasters, Kurland added.

“What is irreplacable are two guys like this who’ve got good ears,” he said. “They’re the filters, the curators. To see their roles diminished somehow, that’s a loss.”

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